Hardcopy of pybind11 instead of using git submodules (#552)

* removed pybind as submodule
* added hardcopy of pybind11 2.10.0
* rename pybind11 folder to avoid conflicts when changing branch

Co-authored-by: Dhanya Thattil <dhanya.thattil@psi.ch>

libs/pybind/tests/test_embed/external_module.cpp

@@ -0,0 +1,20 @@
+#include <pybind11/pybind11.h>
+
+namespace py = pybind11;
+
+/* Simple test module/test class to check that the referenced internals data of external pybind11
+ * modules aren't preserved over a finalize/initialize.
+ */
+
+PYBIND11_MODULE(external_module, m) {
+    class A {
+    public:
+        explicit A(int value) : v{value} {};
+        int v;
+    };
+
+    py::class_<A>(m, "A").def(py::init<int>()).def_readwrite("value", &A::v);
+
+    m.def("internals_at",
+          []() { return reinterpret_cast<uintptr_t>(&py::detail::get_internals()); });
+}